Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers so as to define a p-n junction. When a bias is applied across the p-n junction, holes and electrons are injected into the active layer where they recombine to generate light in a process called injection electroluminescence. Light may be emitted from the active layer through all surfaces of the LED.
As most LEDs are nearly monochromatic light sources that appear to emit light having a single color, light emitting devices or lamps including multiple LEDs that can emit light of different colors have been employed to produce white light. In these devices, the different colors of light emitted by the individual LEDs combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white.
As an alternative to combining individual LEDs to produce light emitting devices having a particular light emission spectrum, phosphors may be used to control the color of light emitted from LEDs. A phosphor may absorb a portion of the light emitted from an LED at a given wavelength and re-emit the light at different wavelength via the principle of photoluminescence. The conversion of light having a shorter wavelength (or higher frequency) to light having a longer wavelength (or lower frequency) may be referred to as down conversion. For example, a down-converting phosphor may be combined with a blue LED to convert some of the blue wavelengths to yellow wavelengths in order to generate white light.
For some applications, it is desirable that substantially all of the light emitted from an LED is converted to longer wavelength light by the phosphor in order to produce light of a desired color. For example, it is of interest to combine blue LEDs with phosphors capable of emitting yellow, amber (orange) or red light for applications such as traffic lights and automotive lights. While LEDs can be designed to emit yellow to red light without phosphor conversion (e.g., AlInGaP LEDs), such devices tend to have temperature-dependent emission properties and lower efficiencies than blue LEDs.
A challenge with phosphor-converted LEDs designed for down conversion of blue light, however, is that some portion of light from the LED may pass through the phosphor without being converted to the desired wavelength. For example, in the case of a phosphor-converted LED that combines a blue LED with a down-converting phosphor for amber light emission, there may be blue light leakage that diminishes the color purity of the emitted light.